In recent years, the areas being developed for crude oil, natural gas, and other energy resources have spread to the North Sea, Siberia, North America, Sakhalin, and other artic regions and, further, the North Sea, the Gulf of Mexico, the Black Sea, the Mediterranean, the Indian Ocean, and other deep seas, that is, areas of harsh natural environments. Further, from the viewpoint of the emphasis on the global environment, natural gas development has been increasing. At the same time, from the viewpoint of the economy of pipeline systems, a reduction in the weight of the steel materials or higher operating pressures have been sought. To meet with these changes in the environmental conditions, the characteristics demanded from line pipe have become both higher and more diverse. Broadly breaking them down, there are demands for (a) greater thickness/higher strength, (b) higher toughness, (c) improved field weldability and accompanying lower carbon equivalents (Ceq), (d) tougher corrosion resistance, and (e) higher deformation performance in frozen areas and earthquake and fault zones. Further, these characteristics are usually demanded in combination in accordance with the usage environment.
Furthermore, due to the recent increase in crude oil and natural gas demand, far off areas for which development had been abandoned up to now due to lack of profitability and areas of harsh natural environments have begun to be developed in earnest. The line pipe used for pipelines for long distance transport of crude oil and natural gas is being required to be made thicker and higher in strength to improve the transport efficiency and also is being strongly required to be made higher in toughness so as to be able to withstand use in artic areas. Achievement of both these characteristics is an important technical goal.
In line pipe in artic zones, fractures are of a concern. The fractures due to the internal pressure of line pipe may be roughly divided into brittle fracture and ductile fracture. The arrest of propagation of the former brittle fracture can be evaluated by a DWTT (drop weight tear test) (which evaluates the toughness of steel in low temperature ranges by the ductile fracture rate and impart absorbed energy at the time of fracture of a test piece by an impact test machine), while the arrest of propagation of the latter ductile fracture can be evaluated by the impact absorbed energy of a Charpy impact test. In particular, in steel pipe for natural gas pipeline use, the internal pressure is high and the crack propagation rate is faster than the speed of the pressure wave after fracture, so there has been an increase in projects seeking not only low temperature toughness (brittle fracture resistance), but also high impact absorbed energy from the viewpoint of prevention of ductile fracture. Achievement of arrest properties of both brittle fracture and ductile fracture is now being sought.
On the other hand, steel pipe for line pipe use may be classified by production process into seamless steel pipe, UOE steel pipe, electric resistance welded steel pipe, and spiral steel pipe. These are selected in accordance with the application, size, etc. With the exception of seamless steel pipe, in each case, flat steel sheet or steel strip is shaped into a tube, then welded to obtain a steel pipe product. Furthermore, these welded steel pipe can be classified by the type of steel sheet used as material. Hot rolled steel sheet (hot coil) of a relatively thin sheet thickness is used by electric resistance welded steel pipe and spiral steel pipe, while thick-gauge sheet material (sheet) of a thick sheet thickness is used by UOE steel pipe. For high strength and large diameter, thick applications, the latter UOE steel pipe is generally used. However, from the viewpoint of cost and delivery, electric resistance welded steel pipe and spiral steel pipe using the former hot rolled steel sheet as a material are advantageous. Demand for higher strength, larger diameter, and greater thickness is increasing.
In UOE steel pipe, the art of production of high strength steel pipe corresponding to the X120 standard is disclosed (see NPLT 1). The above art is predicated on use of heavy sheet as a material. To obtain both high strength and greater thickness, interrupted direct quench (IDQ), a feature of the sheet production process, is used to achieve a high cooling rate and low cooling stop temperature. In particular, to ensure strength, quench hardening (structural strengthening) is utilized.
However, the art of IDQ cannot be applied to the hot rolled steel sheet used as a material for electric resistance welded steel pipe and spiral steel pipe. Hot rolled steel sheet is produced by a process including a coiling step. Due to the restrictions in capacity of coilers, it is difficult to coil a thick material at a low temperature. Therefore, the low temperature cooling stop required for quench hardening is impossible. Therefore, securing strength by quench hardening is difficult.
On the other hand, PLT 1 discloses, as art for hot rolled steel sheet achieving high strength, greater thickness, and low temperature toughness, the art of adding Ca and Si at the time of refining so as to make the inclusions spherical and, furthermore, adding the strengthening elements of Nb, Ti, Mo, and Ni and V having a crystal grain refinement effect and combining low temperature rolling and low temperature coiling. However, this art involves a final rolling temperature of 790 to 830° C., that is, a relatively low temperature, so there is a drop in absorbed energy due to separation and a rise in rolling load due to low temperature rolling and consequently problems remain in operational stability.
PLT 2 discloses, as art for hot rolled steel sheet considering field weldability and excellent in both strength and low temperature toughness, the art of limiting the PCM value to keep down the rise in hardness of the weld zone and making the microstructure a bainitic ferrite single phase and, furthermore, limiting the ratio of precipitation of Nb. However, this art also substantially requires low temperature rolling for obtaining a fine structure. There is a drop in absorbed energy due to separation and a rise in rolling load due to low temperature rolling and consequently problems remain in operational stability.
PLT 3 discloses the art of obtaining ultra high strength steel sheet excellent in high speed ductile fracture characteristics by making the ferrite area ratio of the microstructure 1 to 5% or over 5% to 60% and making the density of (100) of the cross-section rotated 45° from the rolling surface about the axis of the rolling direction not more than 3. However, this art is predicated on UOE steel pipe using heavy sheet as a material. It is not art covering hot rolled steel sheet.